In preparation for moving the `Witness` to `primitives` we need a way to
construct the `Witness` when decoding. In order to maintain the current
performance and not introduce additional allocations we need to be able
to construct a `Witness` from the content buffer, this leaks the
implementation details of `Witness`.
Add a clearly marked unstable constructor to create a `Witness` from
parts. This introduces the concept of `foo__unstable` function names;
add a section to the README describing unstable functions and semver
guarantees.
6ce76cd7c8